Submerged membrane supported bioreactor for conversion of syngas components to liquid products

ABSTRACT

A submerged membrane supported bioreactor for anaerobic conversion of gas into liquid products including a plurality of membrane modules having a plurality of hollow fibers, each of the plurality of hollow fibers having a gas permeable hollow fiber wall defining a hollow fiber lumen and an outer surface; a membrane tank for retaining the membrane modules at least partially submerged in a process liquid for formation of a biofilm on the outer surface of the hollow fiber wall by interaction of microorganisms with a process gas and for the production of a liquid product that mixes with the process liquid, wherein the membrane tank retains the membrane modules in a common horizontal plane; a seal between contents of the membrane tank and ambient atmosphere; and a gas supply conduit for communicating the process gas with the hollow fiber lumens of the hollow fibers.

U.S. patent application Ser. No. 11/781,717, filed Jul. 23, 2007, isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

This invention relates to the biological conversion of CO and mixturesof CO₂ and H₂ to liquid products in bioreactors.

DETAILED DESCRIPTION Background

Biofuels production for use as liquid motor fuels or for blending withconventional gasoline or diesel motor fuels is increasing worldwide.Such biofuels include, for example, ethanol and n-butanol. One of themajor drivers for biofuels is their derivation from renewable resourcesby fermentation and bioprocess technology. Conventionally, biofuels aremade from readily fermentable carbohydrates such as sugars and starches.For example, the two primary agricultural crops that are used forconventional bioethanol production are sugarcane (Brazil and othertropical countries) and corn or maize (U.S. and other temperatecountries). The availability of agricultural feedstocks that providereadily fermentable carbohydrates is limited because of competition withfood and feed production, arable land usage, water availability, andother factors. Consequently, lignocellulosic feedstocks such as forestresidues, trees from plantations, straws, grasses and other agriculturalresidues may become viable feedstocks for biofuel production. However,the very heterogeneous nature of lignocellulosic materials that enablesthem to provide the mechanical support structure of the plants and treesmakes them inherently recalcitrant to bioconversion. Also, thesematerials predominantly contain three separate classes of components asbuilding blocks: cellulose (C₆ sugar polymers), hemicellulose (variousC₅ and C₆ sugar polymers), and lignin (aromatic and ether linked heteropolymers).

For example, breaking down these recalcitrant structures to providefermentable sugars for bioconversion to ethanol typically requirespretreatment steps together with chemical/enzymatic hydrolysis.Furthermore, conventional yeasts are unable to ferment the C₅ sugars toethanol and lignin components are completely unfermentable by suchorganisms. Often lignin accounts for 25 to 30% of the mass content and35 to 45% of the chemical energy content of lignocellulosic biomass. Forall of these reasons, processes based on apretreatment/hydrolysis/fermentation path for conversion oflignocellulose biomass to ethanol, for example, are inherently difficultand often uneconomical multi-step and multi conversion processes.

An alternative technology path is to convert lignocellulosic biomass tosyngas (also known as synthesis gas, primarily a mix of CO, H₂ and CO₂with other components such as CH₄, N₂, NH₃, H₂S and other trace gases)and then ferment this gas with anaerobic microorganisms to producebiofuels such as ethanol, n-butanol, hexanol or chemicals such as aceticacid, butyric acid and the like. This path can be inherently moreefficient than the pretreatment/hydrolysis/fermentation path because thegasification step can convert all of the components to syngas with goodefficiency (e.g., greater than 75%), and some strains of anaerobicmicroorganisms can convert syngas to ethanol, n-butanol or otherchemicals with high (e.g., greater than 90% of theoretical) efficiency.Moreover, syngas or syngas components can be made from many othercarbonaceous feedstocks such as natural gas, reformed gas, peat,petroleum coke, coal, solid waste and land fill gas, making this a moreuniversal technology path.

However, this technology path requires that the syngas components CO andH₂ be efficiently and economically dissolved in the aqueous medium andtransferred to anaerobic microorganisms that convert them to the desiredproducts. And very large quantities of these gases are required. Forexample, the theoretical equations for CO or H₂ to ethanol are:6CO+3H₂O→C₂H₅OH+4CO₂6H₂+2CO₂→C₂H₅OH+3H₂O

Thus, 6 moles of relatively insoluble gases such as CO or H₂ have totransfer to an aqueous medium for each mole of ethanol. Other productssuch as acetic acid and n-butanol have similar large stochiometricrequirements for the gases. Furthermore, the anaerobic microorganismsthat bring about these bioconversions generate very little metabolicenergy from these bioconversions. Consequently they grow very slowly andoften continue the conversions during the non-growth phase of their lifecycle to gain metabolic energy for their maintenance.

Many devices and equipment are used for gas transfer to microorganismsin fermentation and waste treatment applications. These numerousbioreactors all suffer from various drawbacks. In most of theseconventional bioreactors and systems, agitators with specialized bladesor configurations are used. In some others such as gas lift or fluidizedbeds, liquids or gases are circulated via contacting devices. Theagitated vessels require a lot of mechanical power often in the range of4 to 10 KW per 1000 gallons—uneconomical and unwieldy for large scalefermentations that will be required for such syngas bioconversions. Thefluidized or fluid circulating systems cannot provide the required gasdissolution rates. Furthermore, most of these reactors or systems areconfigured for use with microorganisms in planktonic form i.e. theyexist as individual cells in liquid medium.

Existing bioreactors are either small scale, unsuitable for large scalemanufacturing processes, or custom designed, increasing manufacturingand installation costs. Submerged membrane modules for wastewatertreatment, such as the Puron™ MBR Module Model PSH-1500 from KochMembrane Systems (Wilmington, Mass.), have been used in water andwastewater treatment for filtration and biological wastewater treatment.Wastewater and sludge is maintained outside a fiber of microporoushydrophilic membrane, and water is drawn into the center of the fiberthrough the microporous hydrophilic membrane to become treated water.Water fills both the shell side and center of the hollow fibers.

To get high yields and production rates the cell concentrations in thebioreactor need to be high and this requires some form of cell recycleor retention. Conventionally, this is achieved by filtration of thefermentation broth through microporous or nonporous membranes, returningthe cells and purging the excess. These systems are expensive andrequire extensive maintenance and cleaning of the membranes to maintainthe fluxes and other performance parameters.

Cell retention by formation of biofilms is a very good and ofteninexpensive way to increase the density of microorganisms inbioreactors. This requires a solid matrix with large surface area forthe cells to colonize and form a biofilm that contains the metabolizingcells in a matrix of biopolymers that the cells generate. Trickle bedand some fluidized bed bioreactors make use of biofilms to retainmicrobial cells on solid surfaces while providing dissolved gases in theliquid by flow past the solid matrix. They suffer from either being verylarge or unable to provide sufficient gas dissolution rates.

Particular forms of membranes have found use in supporting specifictypes microorganisms for wastewater treatment processes. U.S. Pat. No.4,181,604 discloses the use of hollow fiber membranes for wastetreatment where the outer surface of the fibers supports a layer ofmicroorganisms for aerobic digestion of sludge.

It would be desirable to have a modular membrane supported bioreactorand method of use that would overcome the above disadvantages.

SUMMARY OF THE INVENTION

It has been found that contacting syngas components such as CO or amixture of CO₂ and H₂ with a surface of a membrane and transferringthese components in contact with a biofilm on the opposite side of themembrane will provide a stable system for producing liquid products suchas ethanol, butanol, hexanol and other chemicals. Accordingly thisinvention is a membrane supported bioreactor system for conversion ofsyngas components such as CO, CO₂ and H₂ to liquid fuels and chemicalsby anaerobic micorooganisms supported on the surface of membrane. Thegas fed on the membrane's gas contact side transports through themembrane to a biofilm of the anaerobic microorganisms where it isconverted to the desired liquid products.

The instant invention uses microporous membranes or non-porous membranesor membranes having similar properties that transfer (dissolve) gasesinto liquids for delivering the components in the syngas directly to thecells that use the CO and H₂ in the gas and transform them into ethanoland other soluble products. The membranes concurrently serve as thesupport upon which the fermenting cells grow as a biofilm and are thusretained in a concentrated layer. The result is a highly efficient andeconomical transfer of the syngas at essentially 100% dissolution andutilization, overcoming limitations for the other fermentation methodsand fermenter configurations. The syngas diffuses through the membranefrom the gas side and into the biofilm where it is transformed by themicrobes to the soluble product of interest. Liquid is passed in theliquid side of the membranes via pumping, stirring or similar means toremove the ethanol and other soluble products formed; the products arerecovered via a variety of suitable methods.

A broad embodiment of this invention is a bioreactor system forconverting a feed gas containing at least one of CO or a mixture of CO₂and H₂ to a liquid product. The system comprises a bio-support membranehaving a gas contacting side in contact with the feed gas fortransferring said feed gas across the membrane to a biofilm support sidefor supporting a microorganism that produces a liquid product. The feedgas supply conduit delivers feed gas to the membrane system through afeed gas chamber having fluid communication with the gas supply conduitand the gas contact side of the membrane for supplying feed gas to themembrane. A liquid retention chamber in fluid communication with thebiofilm support side of the membrane receives liquid products and aliquid recovery conduit in fluid communication with the liquid recoverychamber recovers a liquid product from the membrane system.

An additional embodiment of the instant invention includes the supply ofdissolved syngas in the liquid phase to the side of the biofilm incontact with that phase. This allows dissolved gas substrate topenetrate from both sides of the biofilm and maintains the concentrationwithin the biofilm at higher levels allowing improved reaction ratescompared to just supplying the syngas via the membrane alone. This maybe accomplished by pumping a liquid stream where the gases arepredissolved into the liquid or by pumping a mixture of liquidcontaining the syngas present as small bubbles using fine bubblediffusers, jet diffusers or other similar equipment commonly used totransfer gas into liquids. The potential added advantage of using thecombined gas and liquid stream is that the additional shear produced bythe gas/liquid mixture may be beneficial in controlling the thickness ofthe biofilm. The advantage of pre-dissolution of the syngas is that verylittle, if any, of the gas is lost from the system so utilizationefficiency is maximized.

Another embodiment of this invention includes the preferential removalof the carbon dioxide (CO₂) gas that is formed in the bioconversionprocess from the syngas using a membrane that selectively permeates CO₂and then returning the syngas enriched in CO and H₂ to the bioreactor.

Yet another embodiment of this invention includes a modular membranebioreactor for anaerobic conversion of gas into liquid productsincluding a plurality of membrane modules having a plurality of hollowfibers, each of the plurality of hollow fibers having a gas permeablehollow fiber wall defining a hollow fiber lumen and an outer surface; amembrane tank for retaining the membrane modules at least partiallysubmerged in a process liquid for formation of a biofilm on the outersurface of the hollow fiber wall by interaction of microorganisms with aprocess gas and for the production of a liquid product that mixes withthe process liquid, wherein the membrane tank retains the membranemodules in a common horizontal plane across which the hollow fibersextend vertically when at least partially submerged in the processliquid; a seal between contents of the membrane tank and ambientatmosphere; and a gas supply conduit for communicating the process gaswith the hollow fiber lumens of the hollow fibers.

Yet another embodiment of this invention includes a bioreaction methodincluding retaining a process liquid in a membrane tank under anaerobicconditions; maintaining a plurality of membrane modules in horizontallyspaced arrangement and at least partially submerged in the processliquid, the membrane modules having a plurality of hollow fibers, eachof the plurality of hollow fibers having a hollow fiber wall defining ahollow fiber lumen and an outer surface; growing a biofilm on the outersurface of the hollow fibers; and passing a process gas into the hollowfiber lumens and through the hollow fiber wall to interact with thebiofilm and generate a liquid product that mixes with the processliquid.

The foregoing and other features and advantages of the invention willbecome further apparent from the following detailed description of thepresently preferred embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the invention, rather than limiting the scope of theinvention being defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing gas diffusing through a porousmembrane into a liquid and details of a porous membrane, non-porousmembrane and composite membrane.

FIG. 2 is a schematic drawing showing a central passage delivering gasto two parallel membrane walls with a liquid phase to the outside ofeach wall.

FIG. 3 is a schematic drawing showing the interior passage of FIG. 2enclosed by the interior surface of the membrane in tubular form withliquid retained to around the membrane circumference.

FIG. 4 is a schematic drawing showing a bioreactor system with gas andliquid circulation.

FIG. 5 is a schematic drawing showing a bioreactor system with multiplebioreactors arranged in series having intermediate carbon dioxideremoval.

FIGS. 6A and 6B are schematic drawings of a one-headed and two-headedmembrane module, respectively, for use in a bioreactor system with gasand liquid circulation.

FIG. 7 is a schematic drawing of a modular membrane supported bioreactorwith one-headed membrane modules.

FIG. 8 is a schematic drawing of another embodiment of a modularmembrane bioreactor with one-headed membrane modules.

FIG. 9 is a schematic drawing of a modular membrane supported bioreactorwith two-headed membrane modules.

FIG. 10 is a schematic drawing of a modular membrane supportedbioreactor with two-headed membrane modules and a hydrostatic tower.

DETAILED DESCRIPTION OF THE INVENTION

Bioconversions of CO and H₂/CO₂ to acetic acid, ethanol and otherproducts are well known. For example, in a recent book concisedescription of biochemical pathways and energetics of suchbioconversions have been summarized by Das, A. and L. G. Ljungdahl,Electron Transport System in Acetogens and by Drake, H. L. and K. Kusel,Diverse Physiologic Potential of Acetogens, appearing respectively asChapters 14 and 13 of Biochemistry and Physiology of Anaerobic Bacteria,L. G. Ljungdahl eds,. Springer (2003). Any suitable microorganisms thathave the ability to convert the syngas components: CO, H₂, CO₂individually or in combination with each other or with other componentsthat are typically present in syngas may be utilized. Suitablemicroorganisms and/or growth conditions may include those disclosed inU.S. patent application Ser. No. 11/441,392, filed May 25, 2006,entitled “Indirect Or Direct Fermentation of Biomass to Fuel Alcohol,”which discloses a biologically pure culture of the microorganismClostridium carboxidivorans having all of the identifyingcharacteristics of ATCC no. BAA-624; and U.S. patent application Ser.No. 11/514,385 filed Aug. 31, 2006 entitled “Isolation andCharacterization of Novel Clostridial Species,” which discloses abiologically pure culture of the microorganism Clostridium ragsdaleihaving all of the identifying characteristics of ATCC No. BAA-622; bothof which are incorporated herein by reference in their entirety.Clostridium carboxidivorans may be used, for example, to ferment syngasto ethanol, n-butanol and/or hexanol. Clostridium ragsdalei may be used,for example, to ferment syngas to ethanol.

Suitable microorganisms and growth conditions include the anaerobicbacteria Butyribacterium methylotrophicum, having the identifyingcharacteristics of ATCC 33266 which can be adapted to CO and used andthis will enable the production of n-butanol as well as butyric acid astaught in the references: “Evidence for Production of n-Butanol fromCarbon Monoxide by Butyribacterium methylotrophicum,” Journal ofFermentation and Bioengineering, vol. 72, 1991, p. 58-60; “Production ofbutanol and ethanol from synthesis gas via fermentation,” FUEL, vol. 70,May 1991, p. 615-619. Other suitable microorganisms include ClostridiumLjungdahli, with strains having the identifying characteristics of ATCC49587 (U.S. Pat. No. 5,173,429) and ATCC 55988 and 55989 (U.S. Pat. No.6,136,577) and this will enable the production of ethanol as well asacetic acid. All of these references are incorporated herein in theirentirety.

The microorganisms found suitable thus far for this invention requireanaerobic growth conditions. Therefore the system will employ suitablecontrol and sealing methods to limit the introduction of oxygen into thesystem. Since the organisms reside principally in contact with theliquid volume of the retention chamber the system maintains a suitableredox potential in the liquid and this chamber may be monitored to makeinsure anaerobic conditions. Anaerobic conditions in the retained liquidvolume are usually defined as having a redox potential of less than −200mV and preferably a redox potential in the range of from −300 to −500mV. To further minimize exposure of the microorganisms to oxygen thefeed gas will preferably have an oxygen concentration of less than 1000ppm, more preferably less than 100 ppm, and even more preferably lessthan 10 ppm.

The instant invention uses microporous membranes or non-porous membranesor membranes having similar properties in being able to transfer(dissolve) gases into liquids for delivering the components in thesyngas directly to the cells that use the CO and H₂ in the gas andtransform them into ethanol and other soluble products. The membranesconcurrently serve as the support upon which the fermenting cells growas a biofilm and are thus retained in a concentrated layer. The resultis a highly efficient and economical transfer of the syngas atessentially 100% dissolution and utilization, overcoming limitations forthe other fermentation methods and fermenter configurations. The syngasdiffuses through the membrane from the gas side and into the biofilmwhere it is transformed by the microbes to the soluble product ofinterest. Liquid is passed in the liquid side of the membranes viapumping, stirring or similar means to remove the ethanol and othersoluble products formed; the products are recovered via a variety ofsuitable methods.

Microporous membranes made from polymers or ceramics have been recentlydeveloped and commercialized for wastewater treatment and purificationapplications. Some variations of these have also been developed foraeration or oxygenation of liquids. Typically these membranes are madefrom hydrophobic polymers such as polyethylene or polypropylene whichare processed to create a fine porous structure in the polymer film.Many commercial organizations supply such membranes primarily in twoimportant geometries—hollow fiber and flat sheets. These can then bemade into modules by appropriate potting and fitting and these moduleshave very high surface area of pores in small volumes.

Suitable hydrophobic microporous hollow fiber membranes have been usedfor degassing applications to remove oxygen, carbon dioxide, and othergases from water and other liquids. An example of commercial membranemodules for such applications is the Liqui-Cel® membrane contactor fromMembrana (Charlotte, N.C.), containing the polypropylene (PP) X40 or X50hollow fibers. CELGARD® microporous PP hollow fiber membrane, containingthe X30 fibers, is also available from Membrana for oxygenationapplications. Liqui-Cel® membrane modules suitable for large scaleindustrial applications have large membrane surface areas (e.g., 220 m²active membrane surface area for Liqui-Cel® Industrial 14×28). Somecharacteristics of these fibers are given in the Table 1 below.

TABLE 1 X30 X40 X50 Porosity (nominal) 40% 25% 40% Pore Size 0.03 μm 0.04 μm  0.04 μm  Internal Diameter 240 μm 200 μm 220 μm Outer Diameter300 μm 300 μm 300 μm Wall Thickness  30 μm  50 μm  40 μm

A microporous PP hollow fiber membrane product (CellGas® module) isavailable from Spectrum Laboratories (Rancho Dominguez, Calif.) forgentle oxygenation of bioreactors without excessive shear to themicrobial or cell cultures. This PP hollow fiber is hydrophobic, with anominal pore size of 0.05 μm and a fiber inner diameter of 0.2 mm.

For the use of hydrophobic microporous membranes for afore-mentionedapplications, it is necessary to properly manage the pressure differenceacross the membrane to avoid formation of bubbles in the liquid. If thepressure difference is greater than a critical pressure, the value ofwhich depends on properties of the liquid and the membrane, liquid canenter the pore (“wetting”) and the gas transfer rate is significantlyimpeded.

To prevent wetting of pores during operations, some composite membraneshave been developed by the membrane suppliers. The SuperPhobic® membranecontactor from Membrana keeps the gas phase and liquid phase independentby placing a physical barrier in the form of a gas-permeable non-porousmembrane layer on the membrane surface that contacts the process liquid.The SuperPhobic® 4×28 module contains 21.7 m² membrane surface area.Another composite hollow fiber membrane with an ultra-thin nonporousmembrane sandwiched between two porous membranes is available fromMitsubishi Rayon (Model MHF3504) in the form of composite hollow fibershaving at 34 m² membrane area per module.

Non-porous (dense) polymeric membranes have been used commercially forvarious gas separation applications. These membranes separate gases bythe selective permeation across the membrane wall. The solubility in themembrane material and the rate of diffusion through the molecular freevolume in the membrane wall determine its permeation rate for each gas.Gases that exhibit high solubility in the membranes and gasses that aresmall in molecular size permeate faster than larger, less soluble gases.Therefore, the desired gas separation is achieved by using membraneswith suitable selectivity in conjunction with appropriate operatingconditions. For example, Hydrogen Membranes from Medal (Newport, Del.)are used in recovery or purification of hydrogen with preferentialpermeation of hydrogen and CO₂. Medal also provides membranes for CO₂removal with preferential permeation of CO₂.

In addition, composite membranes having a thin nonporous silicone layeron the surface of polypropylene microporous hollow fibers have beenfabricated by Applied Membrane Technology, Inc. (Minnetonka, Minn.) andSenko Medical Instrument Manufacturing (Tokyo, Japan) and evaluated forartificial lung applications. See “Evaluation of Plasma Resistant HollowFiber Membranes for Artificial Lungs” by Heide J. Eash et al. ASAIOJournal, 50(5): 491-497 (2004).

Microporous membranes have been used widely in membrane bioreactors forwastewater treatment. Installations are mostly in the submerged membraneconfiguration using hollow fiber or flat sheet membranes for wastewatertreatment. The structure and module configuration of these membranes mayprove particularly useful for the systems of this invention. Themembranes are typically made of poly(vinylidene fluoride) (PVDF),polyethylene (PE), PP, poly(vinyl chloride) (PVC), or other polymericmaterials. The typical pore size is in the range of 0.03 to 0.4 μm. Thetypical hollow fiber outer diameter is 0.5 to 2.8 mm and inner diameter0.3 to 1.2 mm. In these submerged membrane configurations, wastewatercontaining contaminants are fed into a tank and treated water isfiltered through the membrane with a suction pressure applied to thefiltrate side (the lumen side of the hollow fiber or the center of theflat plate) of the membrane. Typically the tank retains multiplemembrane modules submerged without an individual housing. There are anumber of commercial suppliers of membranes for submerged membranebioreactors in wastewater treatment, each with some distinct features inmembrane geometry and module design as described below. These membranegeometries and module designs can be suitable for the instant inventionand are incorporated herein.

For wastewater treatment and biomedical applications in which efficienttransfer of oxygen into an aqueous phase is desired, hollow fibermembranes made of polymethylpentene (PMP) have been used, due to thehigh permeability of PMP for oxygen. These PMP hollow fibers arenon-porous and of either the skinned asymmetric or dense type asdescribed in “Evaluation of Plasma Resistant Hollow Fiber Membranes forArtificial Lungs” by Heide J. Eash et al. ASAIO Journal, 50(5): 491-497(2004) and U.S. Pat. No. 7,118,672 B2.

A hollow fiber membrane SteraporeSUN™, available from Mistubishi Rayon(Tokyo, Japan), is made of PE with modified hydrophilic membranesurface. The hollow fiber has a nominal pore size of 0.4 μm and a fiberouter diameter of 0.54 mm. A SteraporeSUN™ membrane unit ModelSUN21034LAN has a total membrane surface area of 210 m², containing 70membrane elements Model SUR334LA, each with 3 m² membrane area.

Another hollow fiber membrane SteraporeSADF™ is available fromMitsubishi Rayon. This membrane is made of PVDF with a nominal pore sizeof 0.4 μm and a fiber outer diameter of 2.8 mm. Each SteraporeSADFmembrane element Model SADF2590 contains 25 m² membrane surface area,and each StreraporeSADF™ membrane unit Model SA50090APE06 containing 20SADF2590 membrane elements has a total membrane surface area of 500 m².

Other commercial microporous hollow fiber membranes used for membranebioreactors include but are not limited to the Zenon ZeeWeed® membranesfrom GE Water & Process Technologies (Oakville, Ontario, Canada), thePuron® membranes from Koch Membrane Systems (Wilmington, Mass.), and theMemJet® membranes from Siemens Water Technologies (Warrendale, Pa.).

Kubota Corporation (Tokyo, Japan) markets submerged membrane systems formembrane bioreactors. These membranes are of the flat-plateconfiguration and made of PVC with a pore size of 0.4 μm. Each membranecartridge has 0.8 m² membrane surface area, and a Model EK-400 membraneunit, containing 400 membrane cartridges, has a total membrane area of320 m².

Membranes of the various geometries and compositions described above maybe used in arrangements of unitary arrays or assemblies of variedcomposition in the systems of this invention. Thus bio-support membraneused in the instant invention can be microporous, non-porous, orcomposite membranes or any combination thereof. Any suitable pottingtechnique can be used to collect and provide the necessary assembly ofindividual membrane elements. If microporous, hydrophobic membranes arepreferred due to faster diffusion of gases in the gas-filled pores thanliquid-filled pores.

The feed gas flows through the gas chamber of the membrane unitcontinuously or intermittently. The feed gas pressure is in the range of1 to 1000 psia, preferably 5 to 400 psia, and most preferably 10 to 200psia. Operating at higher gas pressures has the advantage of increasingthe solubilities of gases in the liquid and potentially increasing therates of gas transfer and bioconversion. The differential pressurebetween the liquid and gas phases is managed in a manner that themembrane integrity is not compromised (e.g., the burst strength of themembrane is not exceeded) and the desired gas-liquid interface phase ismaintained.

In such membranes the gas and liquid can be brought into direct andintimate contact without creating any bubbles by operating at adifferential pressure that is below the bubble point of the membraneliquid interface and maintains the gas-liquid interface. Furthermore,the properties of this interface can be controlled by the porosity andhydrophobicity/hydrophlicity properties of the membrane pores.

In this invention, a bio-support membrane suitable for permeation of atleast one of CO or a mixture of H₂ and CO₂ provides the separationbetween a feed gas and a liquid phase. FIG. 1 shows more detail of themembrane configuration and interface in the operation of arepresentative bio-reactor system. FIG. 1( a) depicts syngas stream Aflowing to the gas feed side of the membrane in gas phase maintained ina chamber on the gas contact side of the membrane. The syngas componentsfreely diffuse through the membrane pores to the liquid interface butwithout formation of bubbles. The anaerobic acetogenic bacteria,Clostridium ragsdaeli, having all of the identifying characteristics ofATCC No. BAA-622, is maintained in a fermentation media. Thefermentation media is circulated through a chamber on the opposite sideof the membrane that maintains a liquid volume in contact with theliquid side of the membrane. Suitable microbial cells are present asbio-film on the liquid-contacting side of the membrane surface,converting at least one of CO or H₂/CO₂ in the feed gas to desirableproducts. Since the membrane pores are much smaller than the width ofthe microorganisms they preferentially stay on the membrane surface toconvert CO and H₂/CO₂ to gain metabolic energy, grow and form a biofilmon the membrane surface. A stream B withdraws the liquid phasecomponents from a liquid volume retained about the outer surface of thebiofilm.

FIGS. 1( b)-(c) show various forms of the membrane with a biofilmpresent on the liquid contacting side of the membrane. The membraneportions of FIGS. 1( a) and 1(b) both schematically show a cross-sectionof porous membrane to the left with a biofilm layer developed on theopposite side of the membrane. The interface between the biofilm and themembrane functions as equilibrium partitioning to keep the liquid andgas phases separated from each other. FIG. 1( c) depicts a similararrangement however this time with a nonporous membrane to the left anda biofilm adhering to the surface on the right-hand side of themembrane. FIG. 1( d) illustrates a composite structure for the membranethat positions a porous membrane surface in contact with the gas phasecomponents. The opposite face (right side) of the porous membraneretains a nonporous membrane layer and a biofilm layer adheres to thesurface on the right side of the non-porous membrane layer.

FIG. 2 depicts a generalized view of a typical flow arrangement forefficient use of space in a membrane system. Syngas components enter thesystem as gas stream A and flow into a central space between twomembrane walls. Gas phase contact surfaces of the opposing membranewalls form a distribution chamber for receiving gas from stream A. Gaspermeates simultaneous through, in this case, the porous membrane forconsumption by the microbes in the biofilm layers that adhere to theouter walls of the two opposing membranes. In this manner each gaschannel serves multiple membrane surfaces and the stream B of liquidproducts is delivered from multiple membrane walls. The arrangement ofFIG. 2 can use a flat sheet configuration and be particularly useful forgood flow control and distribution on the liquid side that may benecessary for biofilm thickness control.

FIG. 3 shows the special case of FIG. 2 wherein the opposite wall of thecentral distribution chamber wrap around in continuous form to provide atubular membrane. In this case gas stream A enters the lumen of themembrane and streams B of liquid products flow away from the outer wallsin all directions. Hollow fibers are particularly useful for suchbioreactor configuration.

FIG. 4 illustrates a specific configuration of one embodiment of thisinvention. A gas supply conduit delivers a feed gas Stream 10 containingCO, H₂, and CO₂ at a rate recorded by a flow meter 11. A feed gasdistribution chamber 13 receives the feed gas stream and distributes thefeed to the lumens of tubular membranes in a membrane unit 15 thatprovides a membrane supported bioreactor. A collection chamber 17collects a portion of the feed gas that exits the lumens and an exhaustgas stream 12 from chamber 17 exits the membrane unit.

A tank surrounds the outside of the tubular membrane elements in themembrane supported bioreactor and retains a liquid for growth andmaintenance of a biofilm layer on the outer surface of the membrane. Thetank provides the means of temperature and pH controls for the liquid,which contains nutrients needed to sustain the activity of the microbialcells. The liquid in the tank is stirred to provide adequate mixing andsparged with a suitable gas, if necessary, to maintain a suitablegaseous environment. A re-circulating liquid loop, consisting of Streams14, 16, and 18 re-circulates liquid through the tank. Liquid flows fromthe tank through lines 14 and 16 while line 20 withdraws liquid andtakes to product recovery to recover liquid products. Line 18 returnsthe remaining liquid from line 16 to the tank via pump 19 at raterecorded by flow meter 21. The product recovery step removes thedesirable product from Stream 20, while leaving substantial amounts ofwater and residual nutrients in the treated stream, part of which isreturned to the bioreactor system via line 22. A nutrient feed is addedvia line 24 is added, as needed, to compensate for the amount of waterremoved and to replenish nutrients. Chamber 23 provides any mixing ofthe various streams and for return to the tank via line 18.

The flow rates of Streams 18 and 14, recirculated through the membraneunit, are selected so that there is no significant liquid boundary layerthat impedes mass transfer near the liquid-facing side of the membraneand there is no excessive shear that may severely limit the attachmentof cells and formation of the biofilm on the membrane surface. Thesuperficial linear velocity of the liquid tangential to the membraneshould be in the range of 0.01 to 20 cm/s, preferably 0.05 to 5 cm/s,and most preferably 0.2 to 1.0 cm/s. In addition to the liquid linearvelocity, the biofilm thickness can be controlled by other means tocreate shear on the liquid-biofilm interface, including scouring of theexternal membrane surface with gas bubbles and free movement of thehollow fibers. Also, operating conditions that affect the metabolicactivity of the microbial cells and the mass transfer rates of gases andnutrients can be manipulated to control the biofilm thickness. Thebiofilm thickness in the instant invention is in the range of 5-500 μm,preferably 5-200 μm.

Depending on the nature of the desired product, there are a number oftechnologies that can be used for product recovery. For example,distillation, dephlegmation, pervaporation and liquid-liquid extractioncan be used for the recovery of ethanol and n-butanol, whereaselectrodialysis and ion-exchange can be used for the recovery ofacetate, butyrate, and other ionic products.

In all the depicted arrangements, the CO and H₂ from the syngas areutilized and a gradient for their transport from the gas feed side iscreated due to biochemical reaction on the membrane liquid interface.This reaction creates liquid fuel or chemicals such as ethanol andacetic acid which diffuse into the liquid and are removed viacirculation of the liquid past the biofilm. Thus the very large surfaceareas of the membrane pores are usable for gas transfer to the biofilmand the product is recovered from the liquid side. Furthermore, thereaction rate, gas concentration gradient and the thickness of thebiofilm can be maintained in equilibrium because the microorganisms inthe biofilm will maintain itself only up to the layer where the gas isavailable.

The membranes can be configured into typical modules as shown in FIG. 4for hollow fibers. The gas flows in the fine fibers that are bundled andpotted inside a cylindrical shell or vessel through which the liquid isdistributed and circulated. Very high surface areas in the range of 1000m² to 5000 m² per m³ can be achieved in such modules.

The bioreactor modules can be operated multiple stages of fermentationusing the modules in counter-current, co-current or a combinationthereof mode between the gas and the liquid. In the arrangement as shownin FIG. 4 a counter current operation is depicted.

During the bioconversion excess CO₂ is generated and this gas candiffuse back and dilute out the concentrations of CO and H₂ in the feedgas and thus reduce their mass transfer rates. Other types of membranesthat preferentially permeate CO₂ over CO and H₂ can be used in themulti-stage configuration as shown as an example in FIG. 5 where, usinga membrane that selectively permeates CO₂ and then returning the syngasenriched in CO and H₂ to the bioreactor can be achieved.

FIG. 5 depicts a system where the entering feed gas flows intobioreactor 27 via line 26 and serially through bioreactors 29 and 31 vialines 28, 32 and 34. At the same time liquid that contacts the biofilmlayers enters the system via line 38 and flows countercurrently, withrespect to the gas flow, through bioreactors 31, 29 and 27 via lines 40and 42. Liquid products are recovered from the liquid flowing out ofline 44 and gas stream is withdrawn from the system via line 36.Separation unit 33 provides the stream of line 28 with intermediateremoval of CO₂ from the system via any suitable device or process suchas a membrane or extraction step. Interconnecting lines 32 and 34 alsoprovide the function of establishing continuous communication throughall of the lumens of the different bioreactors so that any combinedcollection and distribution chambers provide a continuous flow path.

FIGS. 6A and 6B are schematic drawings of a one-headed and a two-headedmembrane module, respectively, for use in a bioreactor system with gasand liquid circulation. Each membrane module provides a large surfacearea of gas-permeable membranes in the form of microporous and/ornonporous hollow fibers. The hollow fibers are oriented vertically tomaintain the fibers in the desired position and to avoid displacementdue to gravity and buoyancy forces. A number of the membrane modules canbe located in process liquid contained in a closed membrane tank, sothat a very large total membrane surface area can be achieved with asmall number of membrane tanks, simplifying plant design and reducingcosts. The membrane tank can be round, square, rectangular or any othersuitable shape with gas-tight cover plates on the top. In oneembodiment, the tank operates at a headspace pressure of not more than15 psig. The membranes forming the hollow fibers can be gas-permeablemicroporous and/or nonporous membranes. In one embodiment, process gasfills the hollow fiber lumens, while biofilm and process liquid are onthe shell side of hollow fibers. In one embodiment, the membrane modulescan have support structures around the hollow fibers to keep the hollowfibers from collapsing when not filled with process gas. The membranemodules can be designed to provide a desired distribution of flow of theprocess liquid about individual hollow fibers and/or small bundles ofhollow fibers. Those skilled in the art will appreciate that themembrane modules can have any cross section as desired for a particularpurpose, such as round, rectangular, square, or any other cross sectionthat accommodates a desired pitch and/or spacing.

Each of the membrane modules has a number of hollow fibers and each ofthe hollow fibers has a hollow fiber wall defining a hollow fiber lumenand an outer surface. A process gas is disposed in the hollow fiberlumens and a biofilm is disposed on the outer surface of the hollowfibers. Process gas passes through the hollow fiber wall to interactwith the biofilm and generate a liquid product that mixes with theprocess liquid. The process gas can be a synthesis gas (syngas), such asa mix of CO, H₂ and CO₂ with other components such as CH₄, N₂, NH₃, H₂Sand other trace gases, or the like. The biofilm supports a culture, suchas Clostridium ragsdalei, Butyribacterium methylotrophicum, Clostridiumljungdahlii, Clostridium carboxidivorans, combinations thereof, and thelike, which can generate the liquid product from the syngas. The liquidproduct can be ethanol, n-butanol, hexanol, acetic acid, butyric acid,combinations thereof, and the like, depending on the syngas and cultureselected. Those skilled in the art will appreciate that numerouscombinations of syngas and culture can be selected as desired forgenerating a particular liquid product desired. FIG. 6A illustrates aone-headed membrane module. In this arrangement, the membrane module 50includes a number of hollow fibers 52, each having a hollow fiber walldefining a hollow fiber lumen and an outer surface. A gas inlet chamber54 is operably connected to the hollow fibers 52 to provide the processgas to the hollow fiber lumens. The hollow fibers 52 can be potted tothe gas inlet chamber 54 with an epoxy or the like. The free ends 56 ofthe hollow fibers 52 are allowed to move freely. In one embodiment, thefree ends 56 of the hollow fibers 52 can be loosely enclosed in netting(not shown) to facilitate handling of the membrane module 50 duringinstallation or maintenance. In one embodiment, the free ends 56 of thehollow fibers 52 are open-ended, allowing gas flow from the free ends56. In another embodiment, the hollow fibers 52 are closed-ended,preventing gas flow from and liquid backflow into the free ends 56.

FIG. 6B illustrates a two-headed membrane module. In this arrangement,the membrane module 60 includes a number of hollow fibers 62, eachhaving a hollow fiber wall defining a hollow fiber lumen and an outersurface. A gas inlet chamber 64 is operably connected to one end of thehollow fibers 62 to provide the process gas to the hollow fiber lumensand a gas exhaust chamber 66 is operably connected to the other end ofthe hollow fibers 62 to receive the process gas from the hollow fiberlumens. The hollow fibers 62 can be potted to the gas inlet chamber 64and the gas exhaust chamber 66 with an epoxy or the like. A number ofsupport rods 68 connect the gas inlet chamber 64 and the gas exhaustchamber 66 to provide mechanical strength to the membrane module 60,which must withstand forces caused by buoyancy of the hollow fibers,weight of the hollow fibers and biofilm, membrane module handling, andthe like. The length of the hollow fibers 62 can be greater than thedistance between the gas inlet chamber 64 and the gas exhaust chamber 66to give the hollow fibers 62 some slack and freedom to move. In oneembodiment, the hollow fibers have a length equal to 1.015 to 1.15 timesthe distance between the first potted end and the second potted end toproduce slack in the fibers. In another embodiment, the hollow fibershave a length equal to 1.015 to 1.03 times the distance between thefirst potted end and the second potted end to produce slack in thefibers.

FIG. 7, in which like elements share like reference numbers with FIG.6A, is a schematic drawing of a modular membrane bioreactor withone-headed membrane modules. In this embodiment, the membrane modules 50are one-headed membrane modules with gas inlet chambers 54 operablyconnected to the hollow fibers 52 to provide process gas to hollow fiberlumens. The modular membrane bioreactor 90 includes a membrane tank 80,process liquid 82 disposed in the membrane tank 80 and having a liquidsurface 84, and a number of membrane modules 50 connected in paralleland at least partially submerged below the liquid surface 84. A processgas is disposed in the hollow fiber lumens and a biofilm is disposed onthe outer surface of the hollow fibers 52. Process gas passes throughthe hollow fiber wall to interact with the biofilm and generate a liquidproduct that mixes with the process liquid. In this arrangement, thefree ends 56 of the hollow fibers 52 extend above the liquid surface 84into a headspace 86, so the free ends 56 can be open-ended orclosed-ended. The membrane tank 80 retains the membrane modules 50 in acommon horizontal plane across which the hollow fibers 52 extendvertically when at least partially submerged in the process liquid 82. Aseal between the contents of the membrane tank 80 and the ambientatmosphere, formed by the wall of the membrane tank 80 and seals oninlet and outlet connections, maintains an anaerobic atmosphere withinthe membrane tank 80.

A process gas, such as syngas or the like, enters the membrane tank 80through gas inlet 88 and is distributed to the gas inlet chambers 54 ofeach of the membrane modules 50 through a feed gas manifold 89. Theprocess gas is distributed from the gas inlet chamber 54 into the hollowfiber lumen of each hollow fiber 52. As the process gas flows along thelength of the hollow fibers 52, the process gas passes through thehollow fiber wall of the hollow fibers 52 and generates liquid product,such as ethanol or the like, through interaction with the biofilm on theouter surface of the hollow fibers 52. The liquid product mixes into andmoves through the process liquid 82 by diffusion and convection.Residual process gas exits the free ends 56 of the hollow fibers 52 intothe headspace 86 of the membrane tank 80. The gas inlet 88, feed gasmanifold 89, and gas inlet chambers 54 provide a gas supply conduit forcommunicating the process gas with the hollow fiber lumens of the hollowfibers 52.

An overflow conduit 92 at the liquid surface 84 is connected to themembrane tank 80 to receive exhaust gas and the process liquid, and agas/liquid separation tank 94 is connected to the overflow conduit 92 toseparate the exhaust gas from the process liquid. The overflow conduit92 controls the liquid level in the membrane tank 80 at a predeterminedlevel and allows process liquid and exhaust gas to flow from themembrane tank 80 to the gas/liquid separation tank 94, from which theexhaust gas exits the gas/liquid separation tank 94 through an exhaustoutlet 96. In one embodiment, the modular membrane bioreactor furtherincludes a product recovery system serving one or more modular membranebioreactors and operably connected to receive the process liquid fromthe gas/liquid separation tank 94, separate liquid product from theprocess liquid, and return process liquid to the membrane tank 80. Theprocess liquid including liquid product passes from the gas/liquidseparation tank 94 in a recirculation stream 100. At least a portion ofthe recirculation stream 100 is drawn off as product stream 102 forrecovery of the liquid product from the process liquid. Therecirculation stream 100 is mixed with a recycle stream 104 includingfresh liquid and/or recycled broth, i.e., process liquid at leastpartially stripped of the liquid product, to form a feed stream 106 ofprocess liquid which is pumped into the membrane tank 80 at the liquidinlet port 108 of the membrane tank 80. The position of the liquid inletport 108 can be selected to provide a desired distribution of flow ofthe process liquid 82 about individual hollow fibers 52 and/or smallbundles of hollow fibers 52. In one embodiment, a process liquidmanifold (not shown) connected to the liquid return port 108 can be usedto facilitate distribution of the process liquid around each of themembrane modules 50.

FIG. 8, in which like elements share like reference numbers with FIGS.6A and 7, is a schematic drawing of another embodiment of a modularmembrane bioreactor with one-headed membrane modules. In thisembodiment, the membrane modules 50 are one-headed membrane modules withgas inlet chambers 54 operably connected to the hollow fibers 52 toprovide the process gas to hollow fiber lumens. The free ends of thehollow fibers of the membrane module are completely submerged below theliquid surface. The modular membrane bioreactor 190 includes a membranetank 80, process liquid 82 disposed in the membrane tank 80 and having aliquid surface 84, and a number of membrane modules 50 connected inparallel and submerged below the liquid surface 84. In this arrangement,the free ends 56 of the hollow fibers 52 are below the liquid surface 84and do not extend into a headspace 86, so the free ends 56 can beclosed-ended.

The process liquid 82 in the lower part of the membrane tank 80 issaturated with the residual process gas and becomes oversaturated as itmoves upward due to the reduction in the hydrostatic pressure. As aresult, a portion of the dissolved process gas is released to theheadspace, with or without bubble formation. An exhaust outlet 96 abovethe liquid surface 84 is connected to the membrane tank 80 to receiveexhaust gas from the headspace 86. A liquid outlet 92 below the liquidsurface 84 is connected to the membrane tank 80 to receive the processliquid. The liquid outlet 92 controls the liquid level in the membranetank 80. In one embodiment, the modular membrane bioreactor furtherincludes a product recovery system serving one or more modular membranebioreactors and operably to the liquid outlet 92 connected to receivethe process liquid from the liquid outlet 92, to separate liquid productfrom the process liquid, and to return process liquid to the membranetank 80. The membrane tank 80 can be adapted for complete filling by theprocess liquid 82.

FIG. 9, in which like elements share like reference numbers with FIGS.6B and 7, is a schematic drawing of a modular membrane supportedbioreactor with two-headed membrane modules. In this embodiment, themembrane modules 60 are two-headed membrane modules having a gas inletchamber 64 and a gas outlet chamber 66. The gas inlet chamber 64 and thegas outlet chamber 66 are operably connected to the hollow fibers 62 toallow the process gas to flow through the hollow fiber lumens from thegas inlet chamber 64 to the gas outlet chamber 66. The gas outletchambers 66 of the membrane modules 60 can be submerged below the liquidsurface 84.

The modular membrane bioreactor 290 includes a membrane tank 80, processliquid 82 disposed in the membrane tank 80 and having a liquid surface84, and a number of membrane modules 60 connected in parallel and atleast partially submerged below the liquid surface 84. In thisarrangement, the gas outlet chambers 66 are below the liquid surface 84and do not extend into a headspace 86.

A process gas, such as syngas or the like, enters the membrane tank 80through gas inlet 88 and is distributed to the gas inlet chambers 54 ofeach of the membrane modules 60 through a feed gas manifold 89. Theprocess gas is distributed from the gas inlet chamber 64 into the hollowfiber lumen of each hollow fiber 62. As the process gas flows along thelength of the hollow fibers 62, the process gas transfers through themembrane of the hollow fibers 62 and generates liquid product, such asethanol or the like, through interaction with the biofilm on the outersurface of the hollow fibers 62. The liquid product mixes into and movesthrough the process liquid 82 by diffusion and convection. Residualprocess gas exits the hollow fibers 62 into the gas outlet chambers 66,which are connected to an exhaust outlet 96 through an exhaust gasmanifold 291. In one embodiment, the exhaust outlet 96 can be closed tomaximize process gas utilization efficiency, and the modular membranebioreactor will function as described above for the modular membranebioreactor of FIG. 8 in which the hollow fibers have closed ends.

Those skilled in the art will appreciate that the membrane modules 60can be a plurality of axially stacked membrane modules contained in thebioreactor. A channel communicates process liquid from one membranemodule to an adjacent membrane module in the stack. Each of the membranemodules has a gas inlet chamber operably connected to the first pottedend of the membrane module and a gas outlet chamber operably connectedto the second potted end of the membrane module. The gas outlet chamberof at least one membrane module communicates with the inlet chamber ofan adjacent membrane module.

FIG. 10, in which like elements share like reference numbers with FIGS.6B, 7, and 9, is a schematic drawing of a modular membrane supportedbioreactor with two-headed membrane modules and a hydrostatic tower. Thehydrostatic tower retains a column of the process liquid above themembrane tank to provide as a liquid seal between the tank and theambient atmosphere. In this embodiment, the membrane tank 80 of themodular membrane bioreactor 390 further includes a hydrostatic tower 320above the membrane modules 60, the liquid surface 84 being maintainedbelow the headspace 86 in the hydrostatic tower 320. The hydrostatictower 320 maintains the desired pressure in the process liquid 82 at themembrane modules 60. The hydrostatic tower 320 provides additionalhydrostatic pressure in the membrane tank 80 without incurringsignificant increases in the tank wall size or working volume. Thehydrostatic tower 320 can be used to keep the process liquid pressurizedto increase the gas transfer rate and/or to maintain the gas-liquidinterface.

The bioreaction method includes the steps of retaining a process liquidin a membrane tank under anaerobic conditions; maintaining a pluralityof membrane modules in horizontally spaced arrangement and at leastpartially submerged in the process liquid, the membrane modules having aplurality of hollow fibers, each of the plurality of hollow fibershaving a hollow fiber wall defining a hollow fiber lumen and an outersurface; growing a biofilm on the outer surface of the hollow fibers;and passing a process gas into the hollow fiber lumens and through thehollow fiber wall to interact with the biofilm and generate a liquidproduct that mixes with the process liquid. The biofilm can includemicroorganisms such as Clostridium ragsdalei, Butyribacteriummethylotrophicum, Clostridium Ijungdahlii, Clostridium carboxidivorans,combinations thereof, and the like. The membrane module can be aone-headed membrane module and a gas inlet chamber can distribute theprocess gas to the hollow fiber lumens. The bioreaction method canfurther include the step of recovering the liquid product from theprocess liquid. The growing a biofilm can include growing a biofilmsupporting a culture selected from the group consisting of Clostridiumragsdalei, Butyribacterium methylotrophicum, Clostridium Ijungdahlii,Clostridium carboxidivorans, and combinations thereof. The growing of abiofilm in the various embodiments of this invention can include growinga biofilm supporting a culture selected from the group consisting ofClostridium ragsdalei, Butyribacterium methylotrophicum, ClostridiumIjungdahlii, Clostridium carboxidivorans, and combinations thereof. Theanaerobic acetogenic bacteria, Clostridium carboxidivorans has all ofthe identifying characteristics of ATCC no. BAA-624; can be used andthis will enable the production of ethanol, n-butanol and acetic acid.

The anaerobic bacteria Butyribacterium methylotrophicum, has theidentifying characteristics of ATCC 33266 and can be adapted to CO useto enable the production of n-butanol as well as butyric acid. Theanaerobic bacteria Clostridium Ljungdahlii, has the identifyingcharacteristics of ATCC 55988 and 55989 can be used to enable theproduction of ethanol as well as acetic acid.

EXAMPLE

A Liqui-Cel® membrane contactor MiniModule® 1×5.5 from Membrana(Charlotte, N.C.) is used as a membrane supported bioreactor for theconversion of carbon monoxide and hydrogen into ethanol. This membranemodule contains X50 microporous hydrophobic polypropylene hollow fiberswith 40% porosity and 0.04 μm pore size. The fiber outer diameter is 300μm and internal diameter 220 μm. The active membrane surface area of themodule is 0.18 m². A gas containing 40% CO, 30% H₂, and 30% CO₂ is fedto the lumen of the fibers at 60 std ml/min and 2 psig inlet pressureand the residual gas exits the module at 1 psig outlet pressure. Themembrane module is connected to a 3-liter BioFlo® 110 Fermentor from NewBrunswick Scientific (Edison, N.J.). The fermentation medium having thecomposition given in Table 2 is pumped from the fermentor, flows throughthe shell side of the membrane module, and returns to the fermentor. Theflow rate of this recirculating medium is 180 ml/min, and the pressureat the outlet of the membrane module is maintained at 5 psig byadjusting a back-pressure valve. The fermentor contains 2 liters of thefermentation medium, which is agitated at 100 rpm and maintained at 37°C. The fermentor is maintained under anaerobic conditions.

The fresh fermentation medium contains the components listed in Tables 2& 3(a)-(d). Initially, the bioreactor system is operated in the batchmode and inoculated with 200 ml of an active culture of Clostridiumragsdalei ATCC No. BAA-622. The fermentation pH is controlled at pH 5.9in the first 24 hours by addition of 1 N NaHCO₃ to favor cell growth andthen allowed to drop without control until it reaches pH 4.5 to favorethanol production. The system remains in the batch mode for 10 days toestablish the attachment of the microbial cells on the membrane surface.Then, the system is switched to continuous operation, with continuouswithdrawal of the fermentation broth for product recovery and replenishof fresh medium. With the continuous operation, suspended cells in thefermentation broth are gradually removed from the bioreactor system anddecrease in concentration, while the biofilm attached on the membranesurface continues to grow until the biofilm reaches a thicknessequilibrated with the operating conditions. The ethanol concentration atthe end of the 10-day batch operation is 5 g/L. At the beginning of thecontinuous operation, a low broth withdrawal rate is selected so thatthe ethanol concentration in the broth does not decrease but increaseswith time. The broth withdrawal rate is then gradually increased. After20 days of continuous operation, the ethanol concentration increases to10 g/L with the broth withdrawal rate at 20 ml/hr.

TABLE 2 Fermentation Medium Compositions Components Amount per literMineral solution, See Table 2(a) 25 ml Trace metal solution, See Table2(b) 10 ml Vitamins solution, See Table 2(c) 10 ml Yeast Extract 0.5 gAdjust pH with NaOH 6.1 Reducing agent, See Table 2(d) 2.5 ml

TABLE 3(a) Mineral Solution Components Concentration (g/L) NaCl 80 NH₄Cl100 KCl 10 KH₂PO₄ 10 MgSO₄•7H₂O 20 CaCl₂•2H₂O 4

TABLE 3(b) Trace Metals Solution Components Concentration (g/L)Nitrilotriacetic acid 2.0 Adjust the pH to 6.0 with KOH MnSO₄•H₂O 1.0Fe(NH₄)₂(SO₄)₂•6H₂O 0.8 CoCl₂•6H₂O 0.2 ZnSO₄•7H₂O 1.0 NiCl₂•6H₂O 0.2Na₂MoO₄•2H₂O 0.02 Na₂SeO₄ 0.1 Na₂WO₄ 0.2

TABLE 3(c) Vitamin Solution Components Concentration (mg/L)Pyridoxine•HCl 10 Thiamine•HCl 5 Roboflavin 5 Calcium Pantothenate 5Thioctic acid 5 p-Aminobenzoic acid 5 Nicotinic acid 5 Vitamin B12 5Mercaptoethanesulfonic acid 5 Biotin 2 Folic acid 2

TABLE 3(d) Reducing Agent Components Concentration (g/L) Cysteine (freebase) 40 Na₂S•9H₂O 40

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the scope of the invention. The scope of theinvention is indicated in the appended claims, and all changes that comewithin the meaning and range of equivalents are intended to be embracedtherein.

1. A membrane bioreactor for anaerobic conversion of a process gas intoliquid products comprising: a plurality of two headed membrane moduleshaving a plurality of hollow fibers, each of the plurality of hollowfibers having a gas permeable hollow fiber wall defining a hollow fiberlumen and an outer surface, a first potted end spaced apart from asecond potted end, the first potted end operably connected to one end ofthe hollow fibers and the second potted end operably connected to theother end of the hollow fibers to allow the process gas to flow throughthe hollow fiber lumens from the first potted end to the second pottedend; a membrane tank adapted for complete filling by a process liquidand for retaining the membrane modules submerged in a process liquid forformation of a biofilm on the outer surface of the hollow fiber wall byinteraction of microorganisms with the process gas and for theproduction of a liquid product that mixes with the process liquid,wherein the membrane tank retains the membrane modules in a commonhorizontal plane across which the hollow fibers extend vertically; aseal between contents of the membrane tank and ambient atmospherecomprising a hydrostatic tower located above all of the modules andextending above the tank, the membrane modules being in fluidcommunication with the hydrostatic tower for retaining a column of theprocess liquid above the membrane tank to provide a liquid seal betweenthe membrane tank and the ambient atmosphere and to pressurize theprocess liquid that surrounds the membrane modules; and, a gas supplyconduit for communicating the process gas with the hollow fiber lumensof the hollow fibers.
 2. The bioreactor of claim 1 wherein: the gassupply conduit is operably connected to the hollow fibers to supply theprocess gas containing at least one of CO or a mixture of CO₂ and H₂;and, the membrane tank retains membrane modules in the process liquidfor the formation of the biofilm containing microorganisms selected fromthe group consisting of Clostridium ragsdalei, Butyribacteriummethylotrophicum, Clostridium Ijungdahlii, Clostridium carboxidivorans,and combinations thereof and for the production of the liquid productselected from the group consisting of ethanol, n-butanol, hexanol,acetic acid, butyric acid, and combinations thereof.
 3. The bioreactorof claim 1 further comprising: a liquid outlet in communication with themembrane tank to withdraw the process liquid; a pressure reducing valveoperably connected to the liquid outlet to receive the process liquidand reduce to the pressure of the process liquid to diffuse dissolvedprocess gas from the process liquid; and a gas/liquid separation tankoperably connected to the pressure reducing valve to separate thediffused process gas from the process liquid.
 4. The bioreactor of claim3 further comprising a product recovery system operably connected toreceive the process liquid from the gas/liquid separation tank, toseparate the liquid product from the process liquid, and to return theprocess liquid to the membrane tank.
 5. The bioreactor of claim 1wherein the hollow fibers have a length equal to 1.015 to 1.15 times thedistance between the first potted end and the second potted end toproduce slack in the fibers.
 6. The bioreactor of claim 1 wherein thetwo-headed membrane modules are operably connected so that the processgas flows in parallel through the hollow fiber lumens of the pluralityof two-headed membrane modules and the two-headed membrane modules areopen for contact of the outer surface of the hollow fibers with theprocess liquid across the volume of the membrane tank.
 7. The bioreactorof claim 1 wherein the working volume of the hydrostatic tower is lessthan the working volume of the membrane tank.